I admit a keen interest in Uranus. The mass ratios of the larger satellites to Uranus are not all that different from the Galilean satellites to Jupiter. Having two systems so grossly similar , yet with such huge differences, such as the axial tilt and the strength of the magnetic field, are so intriuging.

Would Hubble (or some of the larger ground based installations) be put to good use checking for co-orbital lagrangian moons of the larger satellites? I recall Voyager doing a thorough search closer in to the planet, but could small dark co-orbitals at Titania and Oberon have been missed?

That Jupiter seems to not have them, and Saturn having a few, it makes me all the more curious for a good inspection of Uranus to see where it stands regarding Lagrangian objects.

Finding some at Uranus might bump up the level of scrutiny Cassini is giving them at Dione and Tethys.

Would Hubble (or some of the larger ground based installations) be put to good use checking for co-orbital lagrangian moons of the larger satellites? I recall Voyager doing a thorough search closer in to the planet, but could small dark co-orbitals at Titania and Oberon have been missed?

The four big Uranian moons are at about the same distance from Uranus as Tethys, Dione and Rhea are from Saturn. They also have similar masses. That being the case, you'd think that there would be at least one Lagrangian satellite somewhere in the Uranian system.

I wonder if the lack of co-orbital satellites for Ariel, Umbriel, Titania and Oberon has to do with Uranus itself, rather than the major moons? Uranus only has about 15% of Saturn's mass. Could this cause Lagrangian orbits to be less stable at Uranus?

The four big Uranian moons are at about the same distance from Uranus as Tethys, Dione and Rhea are from Saturn. They also have similar masses.

Interestingly, a stronger comparison exists between Uranus and Jupiter, namely that both planets have similar planet/satellite mass ratios. In fact, in 2003 Andy Heaton and Jim Longuski published a paper showing that a Galileo-style tour is, unlike at Saturn and Neptune, possible at Uranus:

Feasibility of a Galileo-Style Tour of the Uranian SatellitesAndrew F. Heaton and James M. LonguskiJ. Spacecraft Rockets40, 591-596 (2003).First page

Interestingly, a stronger comparison exists between Uranus and Jupiter, namely that both planets have similar planet/satellite mass ratios. In fact, in 2003 Andy Heaton and Jim Longuski published a paper showing that a Galileo-style tour is, unlike at Saturn and Neptune, possible at Uranus:

Feasibility of a Galileo-Style Tour of the Uranian SatellitesAndrew F. Heaton and James M. LonguskiJ. Spacecraft Rockets40, 591-596 (2003).First page

I'll be darned! How 'bout that. I was aware of the mass ratio and distance scaling between Uranus and Jupiter, but I assumed there was an inverse square {or cube} law that prevents a 'quick' style tour of the Uranian moons. I thought I was really 'pushing' things suggesting a tour of the system lasting upwards of 20 years. {btw, a 20 year mission duration at Uranus is desirable for other reasons}

The lack of a strong resonance of three (or more) members of the Uranian system doesn't hurt the mission either. The lower radiation environment at Uranus would certainly be a relief after the ordeal Galileo experienced.

I'll be darned! How 'bout that. I was aware of the mass ratio and distance scaling between Uranus and Jupiter, but I assumed there was an inverse square {or cube} law that prevents a 'quick' style tour of the Uranian moons.

The most important thing for a "Galileo-like" tour is the availability of multiple satellites to utilize gravity assists, which adds flexibility to the tour design process. Cassini, on the other hand, can only use Titan and, consequently, must always return to Titan for a gravity assist to continue a given tour. This makes designing a "Cassini-like" tour very difficult, especially when multiple targets (e.g., Saturns icy satellites) are desired and the mission has severe operational constraints. Just ask the Cassini mission planners what they went though during the design process for the "T9-X" and "T18-X"-class tours.

QUOTE (tasp @ Nov 12 2005, 02:21 PM)

I thought I was really 'pushing' things suggesting a tour of the system lasting upwards of 20 years. {btw, a 20 year mission duration at Uranus is desirable for other reasons}

The lack of a strong resonance of three (or more) members of the Uranian system doesn't hurt the mission either. The lower radiation environment at Uranus would certainly be a relief after the ordeal Galileo experienced.

Agreed.

QUOTE (tasp @ Nov 12 2005, 02:21 PM)

Appreciate the post!

My pleasure.

Trivia: For those who do happen to read the full paper by Heaton and Longuski, one might note that I get a brief mention in the acknowledgements section. I provided a couple of Uranus scientific references to Andy Heaton between the time he presented the paper at an AAS/AIAA conference and when he submitted the final for publication in J. Spacecraft Rockets.

Without incurring copyright snags, of course, could I field a question about the ideas in the paper?

The portion available through the link ends with an intriguiging acknowledgement that the vehicle could enter orbit about Ariel after ~40 loops around Uranus.

Is this characteristic of the orbit tour similar (at least broadly) to the clever trajectory Messenger is utilizing to eventually end up in orbit about Mercury?

I realize the mass ratios of Sun - Mercury - Venus - Earth are quite different, but is there a correlation between the flight plans?

Amazing article, regardless!

Darn, wouldn't you know that I don't have a copy of the paper available on my system at the moment

That said, I'll review it when I get home but from what I recall, the main concept behind the end-of-tour plan to insert into orbit at Ariel is based on orbital pumping and cranking, which not only changes inclination from the initial Uranian insertion, but reduces relative velocity so that the deterministic delta-V for insertion at Ariel can be handled by the orbiter's propulsion system. I can't remember whether the scheme also utilizes any "third-body" effects from Uranus or "fuzzy boundaries" but these might possible as was planned for the original Europa Orbiter insertion in europan orbit.

Whether this resembles, even in a broad sense, the MESSENGER trajectory through the inner solar system, is, I guess, in the eye of the beholder.

I realize with Jupiters small axial tilt, we didn't see too many (any) seasonal effects with Galileo.

With Cassini, if we are lucky and have 7 years of probe life, we get to travel with Saturn roughly 90 degrees around the sun. We can see the rings from nearly full open to the sun to edge on. Assuming the rings look pretty much the same top and bottom (north and south?) we get to see the 'whole show' or at least be able to extrapolate it form a 7 year exploration of Saturn.

At Uranus, same thing. How does the ring look through forward scattering at edge on and full on to the sun? (I'm hoping for some phase effects, perhaps some of the Uranian ring particles are elongated and align radially to Uranus)

Additionally, if a probe arrived at Uranus with Uranian rings 45 degrees short of edge on to the sun (not that orbit for a probe would be easier to achieve in this orientation) and the probe could watch the rings go edge on then to 45 degrees further (in ~20 years), we could also get near 100% coverage in sunlight of all the surfaces of the main satellites.

A probe arrival with the rings full on to the sun, would also be desirable for other reasons.

{granted a probe with an even longer life would be great, but a factor of 2 or 3 increase from Cassini would seem technically possible}

Orbiting Ariel would be a remarkable ending to this mission -- especially given the assumptions I've been making until this month about the helplessness of a Uranus Orbiter. (I might as well say that Spilker told me that his own navigator for the Neptune Orbiter mission design took just a few days to reach a similar conclusion that a moon tour is perfectly feasible with a Uranus Orbiter; the only thing needed is experience with navigating a craft through a rapid-fire series of moon flybys, which Cassini has now given us. He didn't say anything about a finish-up at Ariel, though.)

A long time ago (the early 1970s!), I suggested that there might be all sorts of odd effects around Uranus as a result of it's axial tilt - specifically, that at certain times of the Uranian year the relative velocities of both incoming spacecraft and meteorites, comets etc could be very much slower than expected (not so much a matter of traditional orbital dynamics as simply the summing of vectors). The end result might be a natural cache - as in Antarctica - of almost Kuiper Belt material, available for access in crater/canyon walls or by drilling on some of the icy moons. Or even interstellar material...

Space Enthusiast Richard Hendricks --"The engineers, as usual, made a tremendous fuss. Again as usual, they did the job in half the time they had dismissed as being absolutely impossible." --Rescue Party, Arthur C ClarkeMother Nature is the final inspector of all quality.

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